Method and control device for operating a mill train for metal strip

ABSTRACT

The invention relates to a method and a control device for operating a mill train for metal strip, which comprises at least one roll stand, the intrinsic flatness of the metal strip being determined at the discharge point of the mill train. In order to ensure in a reliable and sufficiently accurate manner that a required visible flatness of the rolled metal strip is kept within predefined limits, the bulging behavior of the metal strip is measured at the discharge point of the mill train and is translated into the intrinsic flatness of thermal strip by means of a bulging model. The visible flatness can thus be better regulated online along the entire mill train by using the bulging mode.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International ApplicationNo. PCT/EP2004/011171, filed Oct. 6, 2004 and claims the benefitthereof. The International Application claims the benefits of GermanPatent application No. 10346274.0 filed Oct. 6, 2003. All of theapplications are incorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The invention relates to a method; one application is particularlysuitable for operation in a hot-rolling mill, e.g. in the finishingtrain, but is not restricted to this.

The invention also relates to a control device.

BACKGROUND OF THE INVENTION

It is known from the unexamined German application DE 198 51 554 A1 thatthe profile and/or flatness of a metal strip is determined at thedischarge point of a mill train and is used to preset a mill train. Themeasured visible flatness is supplied here to a neural network in theform of input parameters.

A flatness regulating system for metal strip is known from DE 197 584 66A1, with a method being employed to measure the surface geometry ofhot-rolled strip by generating lines on the surface of the strip. Thevisible flatness measured in this manner is supplied to a flatnessregulator via a flatness analysis system.

SUMMARY OF THE INVENTION

The object of the invention is to operate a mill train for metal stripsuch that a control is provided to ensure that a required visibleflatness of the rolled metal strip is complied with in a reliable andsufficiently accurate manner within predefined limits.

The object is achieved by a method of the type mentioned above, withvalues for the visible flatness being translated into values for theintrinsic flatness using a bulge model to control the roll stands and amaterial flow model being used to determine the intrinsicflatness—looked at in the material flow direction—before a physicalpoint for measuring flatness.

The claimed possibility of taking into account both the visible flatnessof the mill train and the intrinsic flatness with the aid of the bulgemodel means that extremely stringent requirements can be complied within respect of the quality of the visible flatness of the metal strip,even though the visible flatness or waviness of the metal stripsometimes completely disappears during rolling under tension, i.e.between the roll posts, and cannot therefore be measured in practice inmany instances within the mill train. By translating values for thevisible flatness into values for the intrinsic flatness or values forthe intrinsic flatness into values for the visible flatness, intrinsicstrip flatness values calculated using the material flow model andvisible strip flatness values measured at the discharge point of a milltrain can be brought into line with each other or verified

The bulge model is used first to establish a unique relationship betweenthe intrinsic and visible flatness of the metal strip. It is thenpossible for the first time not just to carry out presettings on thebasis of flatness measurements but also to use the visible flatness foraccurate control or regulation of the ongoing rolling process.

The visible flatness is advantageously determined in the form of a bulgepattern. The bulge pattern is easy to compare in respect of data and canbe stored with relatively little outlay.

The bulge pattern is advantageously three-dimensional.

At least one of the variables wavelength, amplitude and phase offset ofthe individual tracks is advantageously evaluated in addition to therelative length of individual tracks of the metal strip to determine thebulge pattern of the metal strip. The bulge pattern can thus beidentified much more accurately.

A multi-track laser measuring device is advantageously used to determinethe bulge pattern, allowing economical identification of the bulgepattern with a sufficiently high level of precision.

The visible flatness is advantageously measured topometrically. Thismakes surface identification of the surface structure of the strip andin particular of the bulge pattern directly possible.

The flatness values are advantageously translated online. This allowsparticularly precise control or regulation of the strip flatness.

The flatness values are advantageously translated with the aid of anon-line-capable approximation function. This can save on-line computingtime during the translation between visible and intrinsic flatness.

The bulge pattern of the metal strip is advantageously modeled using thebulge model by applying a fictitious temperature distribution in thetransverse direction of the metal strip based on the intrinsic flatnessof the metal strip. The thermal expansion in the longitudinal directionof the strip, but not however in the transverse direction, correspondingto this strip temperature distribution corresponds to a lengthdistribution that can be assigned to the intrinsic flatness. Only onesegment of limited length must therefore be modeled and the modelequations for elastic plate deformations with major deflections can beworked out with suitable edge conditions at the segment edges.

One or more flatness limit values are advantageously predefined atfreely selectable points within and/or after the mill train in order tocontrol the mill train. The flatness limit values can relate to theintrinsic flatness and/or the visible flatness. Because flatness limitvalues can be predefined everywhere within or after the mill train,regulation accuracies for the rolling process can be significantlyincreased.

The object is also achieved by a control device for operating a milltrain for metal strip with at least one roll stand, with the controldevice for implementing a method described above having at least oneregulating unit coupled to a bulge model, which is coupled to a devicefor measuring the visible flatness of the metal strip and to a materialflow model. Advantageous embodiments of the control device are specifiedin the subclaims. The advantages of the control device are similar tothose of the method.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and details will emerge from the description whichfollows of an exemplary embodiment in conjunction with the figures, inwhich:

FIG. 1 shows a multi-stand mill train for rolling metal strip and acontrol device assigned to the mill train,

FIGS. 2 a-2 c show examples of metal strip with flatness errors,

FIG. 3 shows the division of a metal strip into tracks,

FIG. 4 shows a section of a multi-stand mill train with a controldevice,

FIG. 5 shows the geometry of a section of a metal strip.

DETAILED DESCRIPTION OF THE INVENTION

According to FIG. 1 a mill train for rolling a metal strip 1 iscontrolled by a control processor 2. The metal strip 1 can for examplebe a steel strip, an aluminum strip or a non-ferrous metal strip, inparticular a copper strip. The mill train has at least two roll stands3.

The roll stands 3 have at least working rolls 4 and—as shown in FIG. 1for one of the roll stands 3—generally also back-up rolls 5. The rollstands 3 could have even more rolls, for example intermediate rolls thatcan be displaced axially.

The metal strip 1 passes through the mill train in its longitudinaldirection x, with the transverse direction y of the metal strip beinglargely parallel to the axes of the working rolls 4.

The mill train shown in FIG. 1 is configured as a finishing train forhot-rolling steel strip. The present invention is particularly suitablefor use with a multi-stand finishing train for hot-rolling steel stripbut is not restricted to this. The mill train could in particular alsobe configured as a cold-rolling mill train (tandem train) and/or forrolling a non-ferrous metal (e.g. aluminum, copper or anothernon-ferrous metal).

The control device 2 has a regulating unit 11. This in turn has a module10 for profile and flatness control, which is coupled to a material flowmodel 9. The control device 2 predefines target values for profile andflatness control elements (not shown here) to the stand regulators 6.The stand regulators 6 then adjust the control elements according to thepredefined target values.

The input variables supplied to the control device 2 include for examplepass schedule data such as the input thickness of the metal strip 1 anda roll force and draft reduction per pass for each roll stand 3. Theinput variables generally also include an end thickness, a targetprofile value, a target thickness contour and a target flatness patternof the metal strip 1 at the discharge point of the mill train. Therolled metal strip 1 should generally be as flat as possible.

However the metal strip 1 often has flatness errors, as shown by way ofan example and schematically in FIGS. 2 a, 2 b and 2 c. Flatness errorsof the metal strip 1 can be measured at one point x2, as shown in FIG.1, for example using a multi-track laser measuring device 13.

FIG. 2 a shows a centric bulge in the metal strip 1. FIG. 2 b showsflatness errors at the edges of the metal strip 1. FIG. 2 c shows bulgesin the metal strip 1, which occur repeatedly in the longitudinaldirection x of the metal strip 1 and in two areas in particular in thetransverse direction y of the metal strip 1.

The bulges in the metal strip 1 are caused in particular by internalstresses in the metal strip 1. Internal stresses in the metal strip arealso referred to as intrinsic strip flatness ip.

FIG. 3 shows the division of a metal strip 1 into fictitious tracks S1to Sn or into measuring tracks S1′ to Sm′. If the metal strip 1 were tobe cut up into narrow longitudinal strips or into tracks S1 to Sn, itwould be possible to measure an uneven strip length distribution (theintrinsic strip length distribution), which is the cause of the internalstresses in the metal strip 1. The multi-track laser measuring device 13captures the relative length of the metal strip 1 for each measuringtrack S1′ to Sm′ and preferably also determines variables such aswavelength, amplitude and/or the phase offset of the individual tracksS1′ to Sm′. It is important that the associated intrinsic or measuredrelative lengths do not correspond for corresponding fictitious tracksS1 to Sn and measuring tracks S1′ to Sm′.

As shown in FIG. 4, a distinction is made between intrinsic stripflatness ip and visible strip flatness vp when hot-rolling metal strip1. The intrinsic strip flatness ip refers, as mentioned above, to thestrip length distribution over the tracks S1 to Sn. The visible flatnessvp results from the bulge behavior of the strip, which is for example afunction of variables such as strip thickness, strip width, theelasticity module of the metal strip 1 and the overall tension to whichthe metal strip 1 is subjected.

According to FIG. 4 the visible flatness vp is measured at one point x2at the discharge point of the mill train, in particular a finishingtrain, and supplied to a bulge model 12. The visible flatness vp ismeasured according to the invention such that not only is the visiblestrip length distribution over the strip width in the transversedirection y an output variable of a measuring device but thethree-dimensional bulge pattern of the strip can also be reconstructedfrom the measuring device output variables. In the case of a multi-tracklaser measuring system therefore not only the (relative) length of theindividual measuring tracks S1′ to Sm′ is output by the measuring devicebut also wavelength and phase offset for each track S1′ to Sm′. With atopometric measurement of the visible flatness vp the surface structureof the metal strip 1 is captured at the surface and three-dimensionallyover large areas of the metal strip 1. A topometric strip flatnessmeasurement is preferably based on a strip projection method. Strippatterns are thereby projected onto the surface of the metal strip 1 andcontinuously captured with the aid of a matrix camera.

The intrinsic flatness ip is preferably calculated at a point x1 betweenor after the roll stands 3, in particular between and/or after the rollstands 3 of a finishing train. The calculation is thereby preferablymade using a material flow model 9 (see FIG. 1), which is preferablypart of a regulating unit 1. The intrinsic flatness ip calculated by thematerial flow model 9 can be compared with the measured visible flatnessvp with the aid of the bulge model 12 at one point x2 at the dischargepoint of the mill train, at which the visible flatness vp is measured.In the case of a cold-rolling mill in particular it would essentiallyalso be possible to measure the intrinsic flatness ip on the metal strip1.

The bulge model 12 allows a unique relationship to be establishedbetween intrinsic flatness ip and visible flatness vp, as far aspossible. Thus for example with a very thick metal strip 1 with moderateintrinsic lack of flatness it is not possible to conclude the intrinsicflatness ip from the bulge behavior, as such a metal strip 1 generallydoes not bulge.

The various flatness values (ip and vp) are preferably determined in thefollowing sequence:

-   -   1. The visible flatness vp, which generally corresponds to the        bulge behavior of the metal strip 1, is generally measured after        a last roll stand 3, for example at the discharge point of a        finishing train.    -   2. The bulge model 12 is used to determine the intrinsic        flatness ip of the metal strip 1 at the point for measuring the        visible flatness vp (see step 1).    -   3. The material flow model 9 is used to determine the intrinsic        flatness ip between the roll stands 3, for example within the        finishing train. The intrinsic flatness can therefore be        determined before the physical point for measuring flatness, in        this instance intrinsic flatness, looked at in the material flow        direction.

The relationship between an intrinsic flatness ip between the rollstands 3 and an intrinsic flatness ip after the last of the roll stands3 is established using the material flow model 9. Input variables suchas the strip thickness contours of the metal strip 1 as well as flatnesspatterns or flatness values before and after passage through a rollstand 3 can be supplied to the material flow model 9. The material flowmodel 9 determines the intrinsic flatness pattern of the metal strip 1online after passage through the roll stand 3 as well as a roll forcepattern in the transverse direction y of the metal strip 1 and suppliesit to a roll deformation model (not shown in more detail here). The rolldeformation model (not shown in more detail here) is preferably part ofa regulating unit 11. The roll deformation model determines rolldeformations and supplies them to a target value determination unit (notshown in more detail here), which uses the determined roll deformationsand a contour pattern of the metal strip 1 on the stand discharge sideto determine the target values for the profile and flatness controlelements in each individual roll stand 3.

Use of the bulge model 12 allows the material flow model 9 and theprofile and flatness control implemented in the module 10 (see FIG. 1 ineach instance) to be adjusted based on the measured data for visibleflatness vp. Upper and lower limits can be specified for the visibleflatness vp or for the corresponding visible lack of flatness of thestrip and these limits can be translated with the aid of the bulge model12 into limits for the intrinsic flatness ip or intrinsic lack offlatness. The bulge model 12 uses the intrinsic lack of flatness tocalculate the bulge pattern of the metal strip 1. The calculated bulgepattern can be used in turn to determine the visible lack of flatness.Inverse modeling is used for the converse conclusion.

The bulge model 12 is preferably based on the theory of elastic platedeformation. The intrinsic flatness ip is modeled by applying afictitious strip temperature distribution over the strip width, i.e. inthe transverse direction y, resulting in thermal expansion in thelongitudinal direction x of the metal strip 1 and at the same time tothe length distribution associated with the intrinsic flatness ip.

Let us look now at a strip segment of length a, width b and thickness has shown in FIG. 5. The drawing also shows the longitudinal direction x,transverse direction y and a perpendicular z. Only a strip segment witha length a of a half or whole basic bulge length and with periodic edgeconditions at the top and bottom ends of the strip segment is modeled.The edge conditions at the sides of the strip are free edges. The modelequations are partial differential equations and the associated edgeconditions, which can be solved for example using finite differencemethods or finite element methods.

The bulge model 12 can be used directly online as a function of thecomputing time of the solution algorithm. Alternatively an offline modelcan be used to generate an online-capable approximation function, whichis then used online for the bulge model 12.

To understand the mode of operation of the bulge model 12 better, itfirst has to be acknowledged that when hot-rolling a metal strip 1 forexample, the measured deflections of the metal strip 1, which are due tothe bulging of the metal strip 1, are generally significantly largerthan the strip thickness h. They are however typically significantlysmaller than both the typical wavelength of the bulge behavior and alsothe strip width b. While the traditional, linear theory of platedeformation only applies when the deflections are less than or equal toapproximately ⅕ of the strip thickness h, in the present instance anon-linear description of the plate warp must be used. In addition tothe variables shown in FIG. 5, which describe the metal strip 1, theelasticity module or e-module for short is also used, with a constante-module generally being assumed. The non-linear bulge behavior can nowbe described as follows: $\begin{matrix}{{\frac{D}{h} \cdot {\nabla^{4}{w( {x,y} )}}} = {\frac{p}{h} + {L( {{w( {x,y} )},{\Phi( {x,y} )}} )}}} & (I)\end{matrix}$

Forces operating in the plane of the strip are thereby expressed in theform of a potential Φ, also referred to generally as Airy's stressfunction. w refers to the vertical displacement of the metal strip 1while p describes the pressure distribution operating from outside,which acts in the perpendicular z. D is defined by the equation below:$\begin{matrix}{{D\text{:}} = \frac{{Eh}^{3}}{12( {1 - v^{2}} )}} & ({II})\end{matrix}$

E thereby stands for the e-module and v stands for the Poisson's ratioof the metal strip 1.

The following also applies for the term L(w,Φ) from equation (I):$\begin{matrix}{{{L( {w,\Phi} )}\text{:}} = {{\frac{\partial^{2}w}{\partial x^{2}}\frac{\partial^{2}\Phi}{\partial y^{2}}} - {\frac{\partial^{2}w}{\partial y^{2}}\frac{\partial^{2}\Phi}{\partial x^{2}}} - {2\frac{\partial^{2}w}{{\partial x}{\partial y}}\frac{\partial^{2}\Phi}{{\partial x}{\partial y}}}}} & ({III})\end{matrix}$

If assumptions are now made in respect of internal stresses and strainsdue to thermal causes, the following results: $\begin{matrix}{{{\frac{1}{E} \cdot {\nabla^{4}{\Phi( {x,y} )}}} + {K_{x}\frac{\partial^{2}{T( {x,y} )}}{\partial y^{2}}} + {K_{y}\frac{\partial^{2}{T( {x,y} )}}{\partial x^{2}}}} = {{( \frac{\partial^{2}w}{{\partial x}{\partial y}} )^{2} - {\frac{\partial^{2}w}{\partial x^{2}}\frac{\partial^{2}w}{\partial y^{2}}}} = {{- \frac{1}{2}}{L( {{w( {x,y} )},{w( {x,y} )}} )}}}} & ({IV})\end{matrix}$

T thereby refers to the temperature in the metal strip 1 and K_(x) orK_(y) the coefficient of thermal expansion in the longitudinal ortransverse direction (x or y).

The equations (I) and (IV) form a system of two coupled, non-linear,partial differential equations. If suitable edge conditions are nowinserted, for example free edges or periodical edge conditions at thetop and bottom ends of a strip segment, the equations (I) and (IV) canbe solved numerically in an iterative manner.

The basic concept of the invention can be summarized as follows:

The invention relates to a method and a control device for operating amill train for metal strip 1, having at least one roll stand 3, with theintrinsic flatness ip of the metal strip 1 being determined at thedischarge point of the mill train. To ensure compliance with a requiredvisible flatness vp of the rolled metal strip 1 within predefined limitsin a reliable and sufficiently accurate manner, it is proposed that thevisible flatness vp or bulge behavior of the metal strip 1 be determinedor preferably be measured at the discharge point of the mill train andbe translated into the intrinsic flatness ip of the metal strip 1 usinga bulge model 12. The visible flatness can thus be used online with theaid of the bulge model 12 to control the roll stands of the mill train.According to the invention the visible flatness vp can be betterregulated preferably online throughout the mill train with the aid ofthe bulge model 12.

The bulge model 12 is online-capable and establishes a uniquerelationship between the absolute intrinsic flatness ip of the rolledmetal strip 1 and the actual measured visual defects in the metal strip1, in other words the visible flatness vp. It is possible for the firsttime to verify, adjust and coordinate a material flow model 9 based onthe intrinsic flatness or its corresponding profile and flatness controlin respect of the actual measured values.

1-14. (canceled)
 15. A method for operating a metal strip mill train,comprising: determining a desired flatness of the strip via a materialflow model; measuring an actual flatness of the metal strip near adischarge point of the mill train; translating the measured metal stripflatness into flatness values; controlling a roll stand of the milltrain via a bulge model that uses the desired and actual flatness valuesas inputs to reduce the difference between the actual flatness and thedesired flatness of the metal strip.
 16. The method as claimed in claim15, wherein the actual flatness of the metal strip is measured at thedischarge point of the mill train.
 17. The method as claimed in claim15, wherein the actual flatness is determined as a bulge pattern. 18.The method as claimed in claim 17, wherein the bulge pattern isthree-dimensional.
 19. The method as claimed in claim 18, wherein arelative length of individual tracks of the metal strip is evaluated todetermine the bulge pattern along with a variable of the individualtracks selected from the group consisting of: wavelength, amplitude andphase offset.
 20. The method as claimed in claim 19, wherein a lasermeasuring device is used to determine the desired flatness of the metalstrip.
 21. The method as claimed in claim 20, wherein the lasermeasuring device is a multi-track laser measuring device.
 22. The methodas claimed in claim 20, wherein the actual flatness of the metal stripis measured topometrically.
 23. The method as claimed in claim 22,wherein the values for the desired flatness are translated into valuesfor the actual flatness using the bulge model.
 24. The method as claimedin claim 23, wherein the flatness values are translated online.
 25. Themethod as claimed in claim 24, wherein, the flatness values aretranslated online via an approximation function.
 26. The method asclaimed in claim 25, wherein the metal strip bulge pattern based on thestrip flatness is determined via the bulge model by applying an assumedtemperature distribution in the transverse direction of the metal strip.27. The method as claimed in claim 26, wherein the actual flatness ofthe metal strip is measured by a laser measuring device.
 28. The methodas claimed in claim 27, wherein the laser measuring device is amulti-track laser measuring device.
 29. The method as claimed in claim27, wherein a flatness limit value is predefined at points to controlthe mill train.
 30. A metal strip mill train control device, comprising:a device that measures an actual flatness of the metal strip; aregulating unit coupled to a bulge model, the model using a device thatmeasures the actual flatness of the metal strip and a material flowmodel to control a roll stand of the mill train to minimize thedifference between the actual flatness and the desired flatness of themetal strip.
 31. The control device as claimed in claim 30, wherein theactual flatness measuring device is a laser measuring device.
 32. Thecontrol device as claimed in claim 31, wherein the laser measuringdevice is a multi-track laser measuring device.
 33. The control deviceas claimed in claim 31, wherein the bulge model is coupled to atopometric measuring system that determines a bulge pattern of the metalstrip.